Falcon's bad vibes

IN 1997 I bought, as a company car, a new Ford Falcon EL Gli. From the time I took delivery, it had a bad vibration when driven above 80km/h. I took it to Ford to be serviced and each time advised them of the vibration, but they kept telling me it was nothing and that with the then new EL model they changed the design of the engine and the vibration was normal. The engine has always idled roughly. My father has the car and took it back to Ford and they told him they had one other vehicle which had the same issue (notwithstanding they told me it was normal) and they tried to fix that vehicle but to no avail. The head mechanic suggested it might be the torque converter. My father had the torque converter replaced, but the vibration remains. My father is a pensioner and cannot afford to replace bits and pieces of the car until the problem is found, so any help you can give would be appreciated.